I think as long as the PM OP is the only one who can add people to the discussion, I don't see a problem with it.
It hasn't been a problem yet that I am aware of. It can be very useful. However, it is probably best that everyone knows that the OP can do this. Imagine I am telling thenewgreen how I wish b_b would shower after his hockey nights, and then thenewgreen invites him into the PM.